This invention relates to electrophotographic systems utilizing a flexible transparent photoreceptor belt having an image exposure unit on one side of the transparent belt and an image development unit on the opposite side of the transparent belt.
In electrophotographic systems utilizing a flexible photoreceptor belt driven in a continuous path past a series of printing stations having exposure units and development units, the light-emitting elements of the exposure units are focused on the light sensitive surface of the photoreceptor so that the light emitted by each light-emitting element forms a single dot of an image on the photo sensitive surface of the photoreceptor. Because the light converges inwardly toward the image point, variations in the position of the photoreceptor with respect to the light-emitting element can cause blurring or otherwise interfere with the quality of the image. Consequently, a precise spacing must be maintained between the array of light-emitting elements in the image exposure unit and the light sensitive surface of the photoreceptor. In electrophotographic systems in which the photoreceptor is a flexible belt such as one made of organic polymer material there are problems with maintaining a precise set distance between the light-emitting array and the photo sensitive surface of the flexible organic polymer photoreceptor belt which tends to take a set and form a ripple in any portions which are in contact with support members such as drive rollers and the supports for the light emitting arrays when the belt is maintained in a stationary position for any significant period of time.
Attempts have been made to alleviate this problem by mounting the light emitting diode arrays in different ways. For example, U.S. Pat. No. 4,893,926 discloses a correction device which alters the position of the light exposure unit relative to the image holding member so as to allow flexibility in the spacing between the light-emitting elements of the exposure unit and the photoreceptor surface. In another arrangement, disclosed in U.S. Pat. No. 5,612,767, a correction member is provided which changes the position of the light exposure unit with respect to the image holding member, thereby permitting the position of the light exposure unit to be set with respect to the image receiving member.
Japanese Patent No. 2912258 discloses an electrophotographic apparatus in which a flexible photoreceptor in the form of an endless transparent belt having a photosensitive surface driven around a plurality of rollers is guided along cover glasses which engage the belt on one side. Light-emitting diode arrays mounted behind the cover glasses expose the sensitive surface of the photoreceptor after charging to produce a latent electrostatic image, and a developing unit on the opposite side of the belt then develops the latent image. Each cover glass extends across two of the light-emitted diode arrays and provides a planar support surface for the photoreceptor belt at the exposure stations. Any portion of an organic polymer photoreceptor belt which has a ripple because it has taken a set by resting against a support member such as a roller for a period of time, however, cannot be held substantially flat by passing it along such a planar surface at an exposure station. This causes problems in focusing of the light from the diode arrays on the photosensitive surface of the belt
The Haneda U.S. Pat. No. 5,752,137 also discloses an electrophotographic apparatus having a transparent flexible photoreceptor belt driven around a plurality of rollers which passes adjacent to a planar support member provided with exposure scanning slits through which image light from exposure units is transmitted to the photoreceptor.
The Genovese U.S. Pat. No. 5,552,863 discloses an electrophotographic apparatus utilizing a flexible transparent photoreceptor belt driven in an endless loop past a plurality of charging, exposure, and development stations in which the exposure stations focus exposure light through the photoreceptor belt to the photosensitive surface using transparent shoes which slidingly engage the surface of the photoreceptor belt so that the belt is bent at an angle as it passes along each shoe. The development stations are located on the opposite side of the belt in spaces between the exposure shoes where the belt is unsupported.
The prior art exposure and development arrangements do not overcome the problems caused by images formed by light-emitting elements focused on a rippled or changing surface such as an organic polymer photoreceptor belt which has developed a set as a result of resting against support members, or which is inadequately supported in the region in which the image is exposed or developed.